Minerals are naturally occurring, inorganic solids with ordered atomic arrangements and definite chemical compositions. Classifying these geological building blocks provides a fundamental framework for understanding Earth’s composition and diversity.
What Defines a Mineral
To be considered a mineral, a substance must meet five criteria:
- It must be naturally occurring.
- It must be inorganic.
- It must be a solid at Earth’s surface temperatures and pressures.
- It must possess a definite chemical composition.
- It must exhibit an ordered atomic arrangement.
The Primary Classification System: Chemical Composition
The most widely accepted method for classifying minerals relies on their chemical composition. This system categorizes minerals based on the dominant anion or anionic group in their structure. This chemical basis is preferred because the dominant anion strongly influences a mineral’s physical and chemical properties, and its formation conditions. Minerals sharing the same anionic group often exhibit similar characteristics. This classification forms the basis for comprehensive systems used by mineralogists worldwide, such as the Dana and Strunz classifications.
Key Chemical Classes of Minerals
Minerals are divided into several major chemical classes, each defined by its characteristic anionic group.
Silicates form the largest and most abundant class, making up approximately 90% of Earth’s crust. Their building block is the silicon-oxygen tetrahedron (SiO4)4-, where one silicon atom bonds to four oxygen atoms. These tetrahedra link in diverse ways, creating minerals like quartz and feldspar. Mica minerals, such as muscovite and biotite, are also silicates known for their sheet-like structures.
Oxides consist of metal cations bonded with oxygen anions (O2-). These minerals are often important metal ores. Examples include hematite, a source of iron, and magnetite, known for its magnetic properties. Corundum, another oxide, is notable for its hardness and found in gemstones like ruby and sapphire.
Sulfides contain sulfur (S2-) bonded with metals. Many are important metal sources. Pyrite, often called “fool’s gold,” is a well-known iron sulfide. Galena is the primary ore for lead, while sphalerite is the main ore for zinc.
Carbonates feature the carbonate anionic group (CO3)2-. These minerals commonly form in sedimentary environments or through biological processes. Calcite, a major component of limestone and marble, is a recognized carbonate. Dolomite, another carbonate, is often found alongside calcite.
Halides are characterized by a halogen element (fluorine, chlorine, bromine, or iodine) as their dominant anion. They often form through the evaporation of saline waters. Halite, or common table salt, is a prime example. Fluorite, composed of calcium and fluorine, is another common halide.
Native Elements occur in nature as uncombined single elements. This class includes metals like gold, silver, and copper, found in their pure forms. Carbon also forms native element minerals, notably diamond and graphite, which have different properties due to their atomic arrangements.
Sulfates contain the sulfate anionic group (SO4)2-. They frequently form in evaporite deposits or through the oxidation of sulfide minerals. Gypsum, a soft mineral used in construction, is a common sulfate. Barite, a barium sulfate, is another example found in hydrothermal deposits.
Phosphates are defined by the phosphate anionic group (PO4)3-. Although many members are rare, some are significant. Apatite, a group of phosphate minerals, is distributed and a primary source of phosphorus for fertilizers.
Why Mineral Classification is Important
Classifying minerals provides a systematic framework across various scientific disciplines. In geology, it helps scientists understand Earth’s formation processes, geological events, and rock distribution. It is also essential in material science, aiding in identifying minerals with desirable properties for industrial applications, and guides economic geologists in locating and assessing ore deposits crucial for extracting valuable metals and resources. In environmental science, understanding mineral composition can shed light on soil characteristics, water quality, and pollutant behavior in natural systems. This organized approach enables efficient study and discovery, fostering advancements in research and practical applications.